Such a mixer is known from Patent Application EP 0,223,287. The known circuit comprises a first field-effect transistor having its source connected to ground and having its drain connected to an active load. These elements form an inverter stage powered by a direct voltage applied via the active load. The first transistor receives a first signal of radio frequency on its gate.
Said circuit further comprises a branch formed by another field-effect transistor arranged as a variable resistor in series with a capacitance. This branch is arranged between ground and the drain of the first transistor. The other transistor arranged as a variable resistor is controlled by a signal of a second frequency supplied by a local oscillator. The intermediate-frequency mixing result is available on the drain of the first transistor.
The operation of this circuit is based on modulation of the active load of the first transistor. Thus, mixing is achieved by variation of the gain of the first transistor arranged as a linear amplifier, as a result of the variation of the resistance formed by the other field-effect transistor which is biased to obtain zero volt drain-source voltage and which is arranged in the branch in series with a blocking capacitance between ground and the drain of the first inverter transistor.
The known circuit has several disadvantages. First of all, the isolation of the first signal relative to the second signal is not optimum. Moreover, the isolation of the second signal relative to the output signal is not optimum. Finally, the conversion gain of the circuit is low owing to the use of a single amplifier stage.
It is an object of the present invention to provide a circuit having an improved conversion gain. It is another object of the invention to provide a circuit with an effective isolation between the various signals. However, these advantages should not be at the expense of the power consumption of the circuit. Indeed, for the envisaged use in portable receivers the power consumption should be as low as possible. Therefore, the present invention aims at providing a circuit by means of which the above objects are achieved without an increase in power consumption in comparison with the known circuit.